So far as I am aware, Sylviee, the National Curriculum core and foundation subject entitlement applies to special schools as well as mainstream schools unless the National Curriculum has been "disapplied" in part or whole in individual cases and for limited periods of time.
The official document on "disapplication" is downloadable from the Department for Education site:
http://tinyurl.com/6htxmas
If you're looking to defend special school students' entitlement to MFL, I recommend that you have a look at the European Commission report on MFL and SEN at
http://ec.europa.eu/education/languages/pdf/doc647_en.pdf
It's full of good-news stories about MFL learners with SEN in mainstream and special schools. It has a particularly good quote somewhere about the greatest barrier for students with SEN to overcome being the low expectations of certain adults!